This experiment aimed to investigate the effects of in-ovo injection of conjugated linoleic acid (CLA) on growth performance and β-defensin (AvBD1 and AvBD2) genes expression in broilers. A total of 400 fertile eggs (Cobb 500) were randomly assigned to 4 treatments, each having five replicates of 20 eggs. CLA was injected into the air sack on 18 d of incubation (150 and 300 mg/egg; CLA150 and CLA300). Two groups of diluent injected and non-injected were also included as controls. Hatchlings were further evaluated in a 42-d rearing period. Data were analyzed using a completely randomized design. Results showed that the hatchability was not significantly affected by the treatments. The growth performance in the CLA300 group was improved (P < 0.05) compared to both controls. CLA300 increased (P < 0.05) the jejunal villus length on 42 d, leading to a significant increase in villus absorptive surface area (P < 0.05) compared to CLA150 or controls. Abdominal fat weight in the CLA300 group was significantly decreased on 42 d (P < 0.05). The expression of β-defensins was detectable in all groups on 21 and 42 d, irrespective of treatment and age. Differences in AvBD1 gene expression in chicks from different treatments were not significant on 21 d, but the expression level in CLA received groups was more than controls at 42 d (P < 0.05). AvBD2 gene expression in CLA-treated birds was increased compared to controls on 21 d, but only the CLA300 group showed a significant increase compared to both controls on 42 d (P < 0.05). Research highlights: in ovo feeding 300 mg CLA per egg into the air sac decreased abdominal fat pad, improved growth performance and villous absorptive surface area of the jejunum and increased AvBD1 and AvBD2 gene expression in broilers.

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